This invention relates generally to the use of radio signals transmitted by earth orbiting satellites to determine the position of a point in space and more specifically to such a system in the game of golf for determining the distance of a golf ball from a plurality of known positions.
The Global Positioning System (GPS) is a satellite-based radio navigation system capable of determining continuous position, velocity and time information for an unlimited number of users. The NAVSTAR (NAVigation Satellite Timing and Ranging) GPS is an absolute positioning system capable of providing accurate three dimensional position information. When completed, the GPS satellite constellation will comprise 24 satellites distributed throughout six orbital planes equally spaced around the equator and inclined at an angle of 55 degrees. The satellites will orbit at an altitude of approximately 11,000 nautical miles and have an orbital period of 12 sidereal hours. This design ensures that signals from at least four GPS satellites can be received at any point on or above the earth's surface at any point in time. A discussion of GPS technology and applications is given in Harris and Sikorski, GPS Technology and Opportunities, presented at Expo Comm China '92, Beijing, China, Oct. 30-Nov. 4, 1992.
A single receiver of GPS signals is capable of determining receiver absolute positioning in terms of latitudinal, longitudinal and altitudinal coordinates. Two or more such receivers can be used to determine the relative position between the receivers in real time. This method is known as differential positioning and is much more accurate than absolute positional sensing since the errors associated with the absolute positioning are substantially cancelled when one absolute position is subtracted from a second absolute position, provided that the measured distance is small compared to the distance between the receivers and the satellites.
Researchers have recently developed systems utilizing GPS technology to provide highly accurate differential positioning. Examples of such differential positioning systems for surveying applications are given in U.S. Pat. Nos. 5,077,577, 5,148,179, 5,155,490 and 5,194,871. U.S. Pat. No. 5,077,557 discloses a surveying instrument with a global positioning receiver for use in situations wherein a direct line of sight to GPS satellites is obscured in part, thereby preventing complete utilization of the GPS system. The instrument is operated from a remote measuring point and aimed at the point to be measured with a sighting device. The vertical setting of the instrument is monitored by a vertical sensor, thus enabling the instrument to determine the three coordinates of the location to be surveyed. U.S. Pat. No. 5,148,179 discloses a method for accurately determining the position of a roving signal receiver relative to the position of a reference receiver whose position is known with sufficient accuracy. U.S. Pat. No. 5,155,490 discloses a geodetic surveying system using at least three GPS base stations, each having a satellite receiver operating in conjunction with a highly accurate clock. Finally, U.S. Pat. No. 5,194,871 discloses a method and system for accurately measuring the distance between a pair of survey marks by separating the received GPS signals into upper and lower side band components. These separate components are filtered, converted to digital form, multiplied together and then digitally analyzed to determine the power and phase of the carrier signal received from each satellite. Differences in Doppler shift are utilized to distinguish the carrier signals of the different satellites. The power and phase measurements of each of the carrier signals are then further processed to obtain highly accurate coordinates for the locations being surveyed.
Two examples of GPS systems developed for the aircraft industry are U.S. Pat. Nos. 4,894,655 and 5,101,356. U.S. Pat. No. 4,894,655 discloses a GPS landing assistance system comprising a fixed receiver station located in the vicinity of the landing zone and an on-based receiving station located in each air craft. The fixed receiving station contains a computing device for supplying correction data representing the deviation between the estimated receiver position and a reference position. The on-board receiving station contains a receiver for receiving GPS signals and a separate receiver for receiving correction data from the fixed receiving station, wherein an estimated distance between the aircraft and the landing zone can be adjusted with the correction data to provide more accurate position information. U.S. Pat. No. 5,101,356 discloses a moving vehicle attitude measurement system utilizing three GPS receivers coupled to phase comparators to establish the phase difference between the RF carrier signals of the three possible pairs of receivers, to calculate a true attitude value of roll, pitch and yaw.
When extremely accurate distance information is required, the aforementioned systems provide the necessary embodiments to achieve such accuracy but can be cost prohibitive to an average consumer. When less accurate distance information is required, such as for shot distance and distances remaining to a green or pin location in the game of golf, a compact, portable and low cost GPS-based measuring apparatus would be extremely desirable. Because of the non-critical nature of the accuracy of distance information in the game of golf, such a device need only be accurate to within a few meters. Moreover, since there is no need to know such information until after the golfer reaches the location of the played shot, the measuring apparatus need not provide real-time measurement data. Thus, a low-cost system, accurate to within a few meters, capable of measuring the distance between a particular ball position and known reference positions would be an ideal diagnostic tool for the game of golf.